In recent years, 100 Gigabit-per-second (Gbps) long-distance optical transmission has been implemented by dual-polarization quadrature phase-shift keying (DP-QPSK) using a digital coherent technology. To further improve transmission capacity, greater-level modulation schemes such as polarization division multiplexed 16 quadrature amplitude modulation (16-QAM) are being developed. Demand for downsizing optical transceivers is also increasing. At present, lithium-niobate (LiNbO3) Mach-Zehnder (MZ) modulators are used typically as optical modulators. In order to realize downsized DP-QPSK or DP-16QAM transmitters, semiconductor Mach-Zehnder modulators are desired.
There is an intrinsic problem in semiconductor optical modulators in that the modulation characteristic (i.e., the relationship between applied voltage and amount of optical phase rotation, or the voltage to phase change characteristic) varies depending on the wavelength of a light beam input to the modulator. In semiconductor optical modulators, the absorption edge wavelength of the semiconductor material changes according to applied voltage, and the phase of light is modulated making use of the phase shift due to absorption based on Kramers-Kronig relations. Hence, semiconductor optical modulators have wavelength dependency such that the closer to the absorption-edge-wavelength the light to be modulated is, the greater the optical phase change with respect to the voltage change becomes.
On the other hand, because the absorption edge wavelength of a semiconductor optical modulator changes in response to a change in substrate bias voltage, the modulation characteristic can be controlled. In this context, a “substrate bias voltage” is a direct-current (DC) bias voltage for controlling a modulator operating point (which voltage corresponds to a center voltage of a high-frequency electric signal for driving the optical modulator). The substrate bias voltage is distinguished from other types of bias voltages. Other types of bias voltages include an optical phase bias voltage for controlling a phase difference between light beams propagating through the two optical waveguides of a Mach-Zehnder interferometer, and a π/2 shift bias voltage for adjusting the optical phase difference between two Mach-Zehnder interferometers to π/2 radians when performing quadrature (or orthogonal) phase shift keying.
To address the wavelength dependency of the modulation characteristic of semiconductor optical modulators, several techniques for controlling a substrate bias voltage or amplitude of a modulator drive signal according to the wavelength of input light are proposed. The first technique is to set the substrate bias voltage to a predetermined level according to the wavelength, whereby the modulator can be driven at a constant amplitude of a drive signal regardless of a change in wavelength. See, for example, Japanese Laid-open Patent Publication No. 2005-326548 A.
The second technique is to perform feedback control on the substrate bias voltage or drive signal amplitude so as to set the optical phase modulation depth to 100%. A low frequency signal is superimposed on driving data signals and output light signals are monitored. Responsive to the monitoring result, at least one of the substrate bias voltage and the amplitude of the modulator drive signal is/are controlled. See, for example, Japanese Laid-open Patent Publication No. 2012-257164 A.
To deal with temperature change in optical modulators, a Peltier device is generally used to maintain the temperature of optical modulators constant.
In a semiconductor Mach-Zehnder modulator, the voltage to phase change characteristic may undergo change over time or aging. Driver circuits (or drive amplitudes) to drive the respective optical waveguides of an MZ interferometer may also undergo change over time.
Meanwhile, there may be a case in which the optical phase modulation depth is set to an arbitrary level under 100%, depending on a modulation scheme employed. However, the second technique described above postulates the modulation depth of 100% and is unsuitable to expand its control scheme to an arbitrary modulation depth.
Accordingly, there is a demand for an optical transmitter and a technique for controlling an optical modulator that can maintain the optical phase modulation index constant at a desired depth even in the environment where the operating characteristic of the optical modulator varies.